Cell Cycle & Cell Division | Mitosis, Meiosis, Phases, Regulation

Introduction to Cell Cycle

The cell cycle is a series of events that leads to the formation of new cells from existing ones. It is essential for growth, repair, and reproduction in multicellular organisms and includes two main stages:

1. Interphase: Phase of cell growth and DNA replication.

2. M Phase (Mitotic Phase): Includes both nuclear division (karyokinesis) and cytoplasmic division (cytokinesis).

The duration and regulation of these stages vary between cell types and are controlled by molecular checkpoints to prevent errors like uncontrolled growth or genetic mutations.

Phases of the Cell Cycle

1. Interphase
The longest phase, where the cell prepares for division. It includes three sub-phases:

• G1 Phase (Gap 1):

  • Cell grows in size

  • Synthesizes proteins and organelles

  • Highly metabolically active

  • Checkpoint ensures cell is ready for DNA synthesis

• S Phase (Synthesis):

  • DNA replication occurs

  • Chromosome number remains the same, but each chromosome now has two sister chromatids

  • Centrosome also duplicates

• G2 Phase (Gap 2):

  • Further growth

  • Synthesis of microtubules for mitotic spindle

  • Final checkpoint for DNA errors before division

Mnemonic to Remember:
Go Santa Go Merry
G1 – S – G2 – M (sequence of cell cycle)

2. Mitotic Phase (M Phase)

Includes both nuclear and cytoplasmic division. It is divided into:

Mitosis (Karyokinesis) – Produces two identical daughter cells

Phases of Mitosis:

Prophase

• Chromatin condenses into visible chromosomes

• Each chromosome has two sister chromatids joined at the centromere

• Nucleolus disappears

• Spindle fibers begin to form

• Centrosomes move to opposite poles

Mnemonic: Pretty Penguins Make A Toast
Prophase – Prometaphase – Metaphase – Anaphase – Telophase

Prometaphase

• Nuclear envelope breaks down

• Spindle fibers attach to kinetochores

Metaphase

• Chromosomes align at the equatorial plate (metaphase plate)

• Best stage for chromosome analysis

Anaphase

• Centromeres split

• Sister chromatids move to opposite poles

Telophase

• Chromatids decondense into chromatin

• Nuclear envelope re-forms

• Nucleolus reappears

• Spindle fibers disappear

Cytokinesis: Division of cytoplasm

• In plant cells: Cell plate formation

• In animal cells: Cleavage furrow formation

3. Meiosis: Reductional Division

Meiosis occurs in germ cells to produce gametes with half the chromosome number. It consists of two successive divisions:

• Meiosis I (Reductional)

• Meiosis II (Equational)

Meiosis I

Prophase I – Longest and most complex phase
Sub-stages: Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis

Mnemonic: Lazy Zebras Play Daily Dominoes

• Leptotene: Chromosomes start condensing

• Zygotene: Synapsis begins, formation of bivalents (homologous chromosomes)

• Pachytene: Crossing over occurs (exchange of genetic material)

• Diplotene: Chiasmata become visible

• Diakinesis: Terminalization of chiasmata, nucleolus disappears

Metaphase I

• Homologous pairs align at equator

• Spindle attaches to centromeres

Anaphase I

• Homologous chromosomes separate, not sister chromatids

• Chromosome number halved

Telophase I and Cytokinesis

• Two haploid cells form

Meiosis II – Like mitosis but occurs in haploid cells

• Prophase II: New spindle forms

• Metaphase II: Chromosomes align at the center

• Anaphase II: Sister chromatids separate

• Telophase II and Cytokinesis: Four genetically different haploid cells formed

Significance of Meiosis

• Maintains chromosome number across generations

• Introduces genetic variation through crossing over and independent assortment

• Essential for sexual reproduction

Mnemonic for Meiosis Phases:
I Pay Money And Taxes Twice
Interphase – Prophase – Metaphase – Anaphase – Telophase (x2 for two divisions)

Regulation of the Cell Cycle

Controlled by Cyclins and Cyclin-Dependent Kinases (CDKs)

• Cyclins: Regulatory proteins whose levels fluctuate during the cell cycle

• CDKs: Enzymes that, when activated by cyclins, phosphorylate target proteins to advance cell cycle

Checkpoints in Cell Cycle

1. G1 Checkpoint: Monitors cell size, DNA integrity

2. G2 Checkpoint: Checks DNA replication and damage

3. M Checkpoint (Spindle Checkpoint): Ensures chromosomes are attached to spindle before separation

If errors are detected, the cell cycle is halted, and repair mechanisms are triggered. If damage is irreparable, the cell may undergo apoptosis (programmed cell death).

Transport Mechanisms During Cell Division

• Spindle fibers made of tubulin are essential for chromosome movement

• Motor proteins like kinesins and dyneins transport chromosomes along microtubules

• Nuclear envelope breakdown and reformation involve vesicular transport systems

• Cytokinesis uses actin-myosin interactions in animals and Golgi vesicle fusion in plants

Comparison: Mitosis vs Meiosis

Key Mnemonics Recap

• Go Santa Go Merry – G1, S, G2, M phases

• Pretty Penguins Make A Toast – Phases of mitosis

• Lazy Zebras Play Daily Dominoes – Prophase I sub-stages

• I Pay Money And Taxes Twice – Meiosis phase sequence

• Check Mates – G1, G2, and M phase checkpoints

Conclusion

The cell cycle and cell division are foundational biological processes vital for growth, development, and reproduction. Mitosis ensures genetic consistency across cells, while meiosis introduces genetic diversity across generations. Each phase, checkpoint, and transport mechanism is tightly regulated to preserve genomic integrity.